Walsall suffered back-to-back league defeats for the first time since February 2015 after losing at relegation-troubled Oldham.
The only goal came on 63 minutes, when Doncaster loanee Curtis Main slotted neatly into the corner from Jonathan Forte’s pull-back.
Jordy Hiwula missed two good chances for the Saddlers either side of half-time, while Sam Mantom and Andy Taylor also went close.
But Jon Whitney’s men were unable to find an equalising goal, with the result seeing them drop from third to sixth in the League One table.
Whitney made two changes to his Walsall team, with Hiwula and Anthony Forde coming in for Rico Henry and Reece Flanagan.
After a quiet, low-key start to proceedings, the visitors fashioned an excellent opportunity to go ahead on 17 minutes.
Romaine Sawyers threaded a precise, defence-splitting pass through to Hiwula, who had made a clever, untracked dart into the box.
But Oldham keeper Joel Coleman was alert to the danger, racing off his line and spreading himself well to block the striker’s right-footed attempt.
John Sheridan’s resurgent Latics, who had won their previous two matches without conceding a goal, created their first chance shortly after.
Tareiq Holmes-Dennis’ whipped left-wing cross bounced through for Main, who hooked a fierce half-volley wide from inside the area.
Taylor then went close for the Saddlers with a curling 25-yard free-kick which whistled just past the angle of post and bar.
Walsall - targeting a fourth win in five - were enjoying the lion's share of possession and doing most of the pressing.
On 32 minutes, Mantom unleashed a powerful left-footed drive from distance which Coleman acrobatically tipped onto the roof of the net.
The home side then threatened through Matthew Palmer, whose long-range free-kick was easily held by visiting stopper Neil Etheridge.
Although the Saddlers had enjoyed the better of a tight first 45 minutes, it was Oldham who should have taken a one-goal lead into the interval.
Paul Downing dallied in possession and was dispossessed by Forte, who rounded Etheridge before firing into the side-netting from a tricky angle.
HALF-TIME: OLDHAM 0-0 WALSALL
Oldham started the second period brightly, with James Wilson heading wide at the back post from Palmer’s set piece.
Chances were proving few and far between in a scrappy second half, although the visitors fashioned a couple of openings.
Demetriou jinked past two defenders before firing over, while Tom Bradshaw flicked Taylor’s cross goalwards but Coleman easily gathered.
Oldham had entered the match in good form, having lost only two of their previous 13 matches, and they went ahead on 63 minutes.
Forte chased a long pass and outmuscled Matthew Pennington before accurately cutting the ball back for Main to finish first-time past Etheridge.
The Latics looked to double their lead, with Mike Jones cutting inside from the left and fizzing an effort wide of the far post.
Walsall suffered a further blow on 72 minutes, when top scorer Bradshaw was forced off through injury and replaced by Jordan Cook.
Facing the prospect of back-to-back away defeats, the Saddlers pressed for an equaliser.
Pennington headed Taylor’s long free-kick into the arms of Coleman, who then held Hiwula’s well-struck drive.
The on-loan striker really should have levelled with ten minutes remaining.
Mantom’s forward ball was laid off by Adam Chambers to Hiwula, who traded passes with Cook before blazing wildly over from 18 yards.
In stoppage time, Demetriou burst into the area but his shot was charged down by Wilson as Oldham held on for victory.
FULL-TIME: OLDHAM 1-0 WALSALL
Etheridge, Demetriou, Downing, Pennington, Taylor, Chambers, Mantom (Morris 84'), Sawyers, Forde (Lalkovic 66'), Hiwula, Bradshaw (Cook 72').
Subs not used: MacGillivray, Henry, Preston, Kinsella.
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